1. Field of the Invention
The invention relates to a mobile radio system comprising at least a first base station, at least a further base station and at least a switching centre.
The invention further relates to a base station, switching equipment and a detection and control arrangement for such a mobile radio system.
2. Discussion of the Related Art
In a mobile radio system there are provided a plurality of base stations for maintaining radio links to mobile subscriber units. A switching centre also referenced switching system in the following is used for establishing connections to the base stations allocated to the switching system. For a handover, the switching system comprises, for example, means which hand over, as required, a connection between the switching system and a mobile subscriber station existing via a first base station (call handover base station) to a second base station (call take-over base station).
Modem mobile radio systems currently comprise many individual radio cells which have each a limited coverage area. Each cell is supplied with radio signals by a base station, while the transmitter power of the base stations and of the mobile subscriber stations is adapted to the size of a radio cell. In this manner the same radio parameters (for example, frequencies, time slots or codecs) can be used in radio cells which are a specific distance apart, without having to take mutual interference into account.
While a base station is connected to mobile subscriber stations that are located inside its cell via a radio channel, the base stations themselves are connected to switching systems, for example, via cable transmission paths. The switching systems on their part are again connected to at least one fixed network, for example, the public telephone network (PSTN=Public Switched Telephone Network, or ISDN=Integrated Services Digital Network). Via the switching systems a mobile subscriber station can be linked to an arbitrary subscriber of the respective network (mobile originating call), or a subscriber of the respective network to an arbitrary subscriber of the mobile radio network (mobile terminating call). In the case of such a link, this may conventionally relate to a call link, but also to a data link, for example, for a facsimile transmission.
The moment a mobile subscriber station leaves the coverage area of a radio cell assigned thereto, there should be provided that the radio link to the mobile subscriber station is taken over by another radio cell i.e. by another base station. The occurrence of such a change of radio cells may be detected, for example, by measuring the signal field strength, the signal-to-noise ratio, the error probability, the distance between base station and mobile subscriber unit and so on. If such a situation is detected, both the radio link is to be handed over from one base station to the take-over base station and the transmission path between a network subscriber and the handover base station is to be switched to the take-over base station.
In a call handover method which is implemented, for example, in a GSM system, first a free radio channel is to be selected by the take-over base station. This channel is announced to the mobile subscriber station, so that the mobile subscriber station can maintain the radio link along the channel announced thereto. In other systems, for example, in DECT (Digital European Cordless Telecommunication), the selection of the new radio channel is made by the mobile station.
When the mobile station is changed to the new radio channel, also the link in the cabled part of the mobile radio system is to be switched over by appropriately driving the switching systems concerned. Then there is the problem that the radio channel and the switching systems are to be switched over substantially simultaneously, so that no perceivable pauses, for example, caused by clicking sounds, or even longer pauses, occur.
For this purpose, there has also been proposed to build up a conferencing circuit of the mobile subscriber with himself, that is, on the one hand, via the base station that hands over the call and, on the other hand, via the base station that takes over the call, so that there is always a link to the mobile subscriber, irrespective of the instant of switching. The cost of such conferencing circuits, however, is considerable. Furthermore, there is the problem that before the radio channel is switched over, the base station taking over the call, or after the radio channel has been switched over, the base station handing over the call, inserts a noise signal into the radio link that does not yet exist or no longer exists. This is tolerable, it is true, for call links, but, in the case of data links, this may lead to errors that can no longer be corrected.
EP 0 544 457 has disclosed a method for handling a call handover from a first base station to a second base station, in which the link from fixed-network subscriber to switching system is first led via a switch specially provided for the call handover before the radio channels are switched over. In preparation of the call handover, this switch completely establishes the connection path to the call take-over base station via the switching system to which the take-over base station is connected, including rendering the new radio channel available to the take-over base station. After this connection has been established, the handover base station announces to the mobile subscriber station that it is to change to the radio channel of the handover base station. Simultaneously, the switch is given the instruction to switch to the new connection already prepared. Subsequently, the previous connection to the take-over base station can be broken off via the switch.
In a special embodiment in which the (call) data are available as compressed time slot signals, the switch is arranged as a unit for time slot exchange. The advantage of the switch lies in the fact that, prior to the actual call handover, all connections are already prepared, and thus for a switch-over to the network only the time slot signals need to be switched over i.e. exchanged. Once an appropriate control signal has arrived at this switch, the switching may be effected without delay. In this manner the period of time in which the connection to and from the mobile subscriber station is dead is to be smaller than 150 ms.
This switching time may be acceptable for call links. For data transmission, however, a switching time of this order of magnitude is no longer acceptable.